It has been well documented that nonhuman animals, particularly pigeons, can classify photographs that are not defined only by a few simple features but contain a particular type of natural object. Categorization is of great relevance for humans and nonhuman animals to cope with the variability and complexity of their environments, allowing them to respond appropriately to new instances after having learned only a few instances from a given category. The advantage of animal studies is to provide insights about nonverbal, pre-linguistic cognitive mechanisms and to uncover the essential roles of human language in the development of cognitive behaviors. This is a review of recent developments of the categorization studies in animals. The topics included are discrimination of artificial categories created by mimicking the supposed structure of natural categories, theoretical issues surrounding the prototype effects shown by animals with artificial polymorphous, prototype, and family-resemblance categories, and learning of functional associations among highly varied members of a category.
